NEVER mind the bitter end - it is the bitter beginning of an infection that triggers an immune response. We know that taste receptors on the tongue detect bitter foods, but there are also identical taste receptors in the upper airway. Noam Cohen at the University of Pennsylvania in Philadelphia and his team think they know why. They grew cell cultures from sinus tissue samples collected from surgical patients, and found that bitter taste receptors in the tissue picked up the presence of Pseudomonas aeruginosa, a bacterium that can cause pneumonia. The sinus tissue responded by producing nitric oxide to kill the invading microbes (Journal of Clinical Investigation, doi.org/jj4). "Certain people have strong innate defences against these bacteria, which is based on their ability to detect bitterness," says Cohen. "Others who don't really 'taste' these bitter compounds have a weakened defence."